The present invention relates generally to electronic devices whose functional length scales are measured in nanometers, and, more particularly, to simple devices used as building blocks to form more complicated structures, and to the methods for forming such devices. Devices both of micrometer and nanometer scale may be constructed in accordance with the teachings herein.
The silicon (Si) integrated circuit (IC) has dominated electronics and has helped it grow to become one of the world""s largest and most critical industries over the past thirty-five years. However, because of a combination of physical and economic reasons, the miniaturization that has accompanied the growth of Si ICs is reaching its limit. The present scale of devices is on the order of tenths of micrometers. New solutions are being proposed to take electronics to ever smaller levels; such current solutions are directed to constructing nanometer-scale devices.
Prior proposed solutions to the problem of constructing nanometer-scale devices have involved (1) the utilization of extremely fine scale lithography using X-rays, electrons, ions, scanning probes, or stamping to define the device components; (2) direct writing of the device components by electrons, ions, or scanning probes; or (3) the direct chemical synthesis and linking of components with covalent bonds. The major problem with (1) is that the wafer on which the devices are built must be aligned to within a small fraction of the size of the device features in at least two dimensions for several successive stages of lithography, followed by etching or deposition to build the devices. This level of control does not scale well as device sizes are reduced to nanometer scale dimensions. It becomes extremely expensive to implement as devices are scaled down to nanometer scale dimensions. The major problem with (2) is that it is a serial process, and direct writing a wafer full of complex devices, each containing trillions of components, could well require many years. Finally, the problem with (3) is that high information content molecules are typically macromolecular structures such as proteins or DNA, and both have extremely complex and, to date, unpredictable secondary and tertiary structures that cause them to twist into helices, fold into sheets, and form other complex 3D structures that will have a significant and usually deleterious effect on their desired electrical properties as well as make interfacing them to the outside world impossible.
Devices of molecular size with switching electric behavior are currently the focus of research world-wide. These devices are viewed as possible replacement to Si-based technologies. In spite of much speculation, however, there are very few experimental results available at this point in time that show some kind of possible switching behavior, the origin of which (intrinsic or extrinsic) is the subject of debate. In previous systems, the switching was either irreversible, or reversibility was achieved on thick molecular films, or switching was volatile.
The above-identified related patent applications are all based on switching of a bi-stable molecule, an example of which is a rotaxane, from one stable state to another stable state by application of an external electric field. Such configuring of the molecular switch by applying an external electric field, which changes the conformation of the molecule and switches it between a low-resistive and a high-resistive state, can be only done on weakly bonded molecular fragments. Usually, it involves the hydrogen bond, where one can overcome the energy barrier of about 0.5 eV (or less). On the one hand, the conformation of stronger bonded molecules is difficult to change, and requires an application of even stronger fields, which in the case of the hydrogen bond are already comparable to atomic fields (xcx9c10+8 to 10+9 V/cm), and can easily damage the device. On the other hand, the barrier of 0.5 eV is insufficient for any reasonable application, since the corresponding lifetime is short with respect to thermal fluctuations destroying the given configuration of the switch. The lifetime xcfx84 is estimated according to the formula       1    τ    =            ω      0        ⁢          exp      ⁡              (                              -                          U              b                                /          kT                )            
where xcfx890xcx9c1 THz is the typical attempt (molecular vibration) frequency, Ub is the energy barrier, k is Boltzmann""s constant, and T is the temperature. For the barrier 0.5 eV at room temperature, one estimates the lifetime as 5xc3x9710xe2x88x924 sec, i.e., extremely short. Thus, one needs much larger barriers (stronger bonds), but they will prevent the device from switching.
There remains a need for a mechanism that permits configuring the molecular switchable devices in the cross-bar geometry while the conformational charge is controlled by weak intramolecular forces (e.g., hydrogen bonding).
In accordance with the present invention, the molecular switchable devices in the cross-bar geometry are configurable while the conformational change is controlled by intramolecular forces that are stronger than hydrogen bonding. The method of the present invention, which is directed to configuring an ensemble of molecular switches in a prescribed manner in cross-bar geometry and then making such configuration either substantially permanent or stable with regard to temperature fluctuations, comprises the steps of:
providing the ensemble of molecular switches in the cross-bar geometry, each molecular switch comprising a pair of crossed wires that form a junction where one wire crosses another at an angle other than zero degrees;
providing the junction with at least one connector species connecting the pair of crossed wires in the junction, the junction having a functional dimension in the nanometer to micrometer range, wherein at least one connector species and the pair of crossed wires forms an electrochemical cell, the connector species comprising a molecule and having at least one active dipole; and
either substantially permanently configuring the connector species where the connector species has one active dipole or stably configuring the connector species where the connector species has more than one active dipole.
There are two approaches that can be taken to achieve the desired configuration. In one approach, the connector species is configured and then an exchange chemical reaction is carried out to make that configuration substantially permanent. The process comprises:
forming the connector species in the junction;
applying an external electric field to the junction to configure the connector species;
replacing a hydrogen atom with a species that forms a bond with a portion of the dipole that is stronger than a hydrogen bond; and
removing the electric field, leaving the switch substantially permanently in this configuration.
In this first approach, the switch may remain switchable, though bonded with associative bonds that are stronger than the hydrogen bond, by utilizing associative bonding such as fluorine to nitrogen, nitrogen to oxygen, and divalent metal to oxygen. Alternatively, the switch may be made permanent by utilizing even stronger bonds, such as sulfur to oxygen covalent bonding.
In another approach, the connector species is stabilized in one of two stable states as follows:
providing a plurality of dipole groups on the connector species, each capable of hydrogen bonding with an adjacent dipole group on the connector species;
forming the connector species in the junction;
applying an external electric field to the junction to configure the connector species; and
removing the electric field, leaving the switch in the stabilized configuration.
In this second approach, the multiple dipole groups ensure that the switch remains in the state into which it was configured until an opposite polarity electric field is applied to switch it again.
The method of the present invention is very general and allows a configuring of arbitrary molecular devices with practically indefinite lifetime ( greater than 10 years at room temperature).
Advantageously, the method of the present invention introduces a method by which the ensemble of molecular switches can be configured in a prescribed way in a cross-bar geometry and then this configuration be made permanent.
The method of the present invention solves the above-noted problem of switching through high energy barriers and allows (i) configuring of the molecular switches and then (ii) making the configured system stable with respect to thermal noise and other external perturbations.